1. Field of the Invention
The present invention relates to organic solvent solutions containing acid-epoxy pressure-sensitive adhesive resins capable of self-cure for increased cohesive strength, the self-curing reaction being catalyzed by a tertiary amine. In another aspect, the invention relates to a process of manufacture of articles containing films of pressure-sensitive resin.
2. Description of the Prior Art
Pressure-sensitive resins are used conventionally in the form of films to provide adhesive bonds between normally non-adhering superstrates and substrates. The films may be prepared by casting from organic solvent solutions and evaporating the solvent.
Constraints are placed on the molecular weight of the pressure-sensitive resin by the viscosity of the solution necessary for ease of coating on commericial coating machines and by the solution concentration required for economical operation. In general, low solution viscosity and high solution concentration are desired in the pressure-sensitive resin solution, and, hence, a resin of relatively low molecular weight is preferred for film casting.
Once a bond has been made by a pressure-sensitive film placed between a substrate and a superstrate, the pressure-sensitive film may be subjected to stress generated by the weight of the superstrate or by dimensional change in the substrate or superstrate caused by thermal expansion, stress relaxation, plasticizer migration or the like. In general, a high molecular weight and a high cohesive strength is desired in the pressure-sensitive resin so that it may resist the stresses induced in the adhesive film. The opposing requirements of low molecular weight resin for ease of coating and high molecular weight resin for load holding ability are conventionally reconciled by the use of crosslinkable resin systems which can be applied at low molecular weight and cured to a high molecular weight.
Pressure-sensitive adhesive resins containing epoxy and acid groups are widely used because of their ability to self-crosslink on drying and heating to produce pressure-sensitive adhesive films with high cohesive strength. However, the rate of curing of these systems is often too slow for drying and curing conditions used in practice and a catalyst for the acid-epoxy reaction must then be added to the solution before coating.
Tertiary amines are widely accepted as catalysts for the acid-epoxy reaction. Generally, however, the curing rate with the majority of tertiary amines is still insufficient for the drying and curing conditions used in practice. One tertiary amine which has proved to be exceptionally effective for the crosslinking reaction is triethylenediamine. Triethylenediamine has the disadvantage, however, of conferring very short pot life to the pressure-sensitive resin solution. In commercial use, pot life determines the useful coating life time of catalyzed batches of pressure-sensitive adhesive solutions. Catalyzed solutions with short pot lives place severe limitations on the usefulness of the material, regardless of the properties of the adhesive. Furthermore, triethylene-diamine causes an interaction between the pressure-sensitive adhesive and silicone release substrates, particularly if the release substrates are not substantially completely cured. This leads to undesirably high adhesion between the adhesive and release surface with the result that the release paper is difficult to remove and the adhesive properties of the pressure-sensitive adhesive film are impaired. There is, therefore, a need in the art for acid-epoxy pressure-sensitive resin solutions containing a tertiary amine which possess adequate pot life which yield adequate rates of cure and which yield cured pressure-sensitive resins with satisfactory release properties from release substrates.